1. Field of the Invention
The present invention relates generally to radio communication effected using time modulated pulses, and in particular wireless communications using time modulated ultrawideband pulses, such as impulse radio. Still more particularly the present invention provides an apparatus and method for effecting synchrony in a wireless communication system. Wireless communication systems, and especially wireless communication systems conveying coded message transmissions, require a close synchrony between receiving stations and received transmitted signals in order that the intelligence contained in the transmitted messages may be properly understood at the receiver station. A key element in establishing such synchrony is the timing system by which appropriate sampling of the received transmitted signal is carried out.
Prior art wireless communication systems have employed frequency-variable devices for establishing the desired synchrony between a receiver station and a received transmitted signal. One example of such a prior art synchronizing device, a voltage-controlled oscillator (VCO), is used for synchronizing with a trigger pulse contained within the received transmitted signal. In order to move the start point of a frame of a receiver signal to align with (i.e., to synchronize with) the frame pattern of the received transmitted signal, the frequency of the output of the VCO is varied by changing the input control voltage provided to the VCO. Synchronizing may occur in two general contexts: acquisitionxe2x80x94when a receiver is seeking to first synchronize with a received signal, and trackingxe2x80x94when a receiver has synchronized with a received signal and seeks to maintain synchrony.
VCO timing control circuits are expensive and their performance yields too slow a response time in adjusting to a received transmitted signal to maintain the desired synchrony. Operational disadvantages caused by such limitations are exacerbated when encountered in connection with such a highly time-dependent wireless protocol as impulse radio communications. The VCO devices available for synchronizing (acquisition and tracking) in wireless communications have an unacceptably low bandwidth capacity and have too much inertia. They are unacceptably slow in their response, largely because of the high xe2x80x9cQxe2x80x9d values incorporated into their designs. Also, VCO devices are too limited in their range of control to meet the highly agile algorithmic needs of ultrawide band systems.
There is a need for an apparatus and method for effecting synchrony in a wireless communication system that is agile in its response to a received transmitted signal.
There is a need for such an agile synchronizing apparatus and method that is less expensive to implement than the presently available voltage-controlled oscillator (VCO) implementations.
2. Related Art
Recent advances in communications technology have enabled an emerging, revolutionary ultra wideband technology (UWB) called impulse radio communications systems (hereinafter called impulse radio).
Impulse radio was first fully described in a series of patents, including U.S. Pat. No. 4,641,317 (issued Feb. 3, 1987), U.S. Pat. No. 4,813,057 (issued Mar. 14, 1989), U.S. Pat. No. 4,979,186 (issued Dec. 18, 1990) and U.S. Pat. No. 5,363,108 (issued Nov. 8, 1994) to Larry W. Fullerton. A second generation of impulse radio patents include U.S. Pat. No. 5,677,927 (issued Oct. 14, 1997) to Fullerton et al; and U.S. Pat. No. 5,687,169 (issued Nov. 11, 1997) and U.S. Pat. No. 5,832,035 (issued Nov. 3, 1998) to Fullerton. These patent documents are incorporated herein by reference.
Uses of impulse radio systems are described in U.S. patent application Ser. No. 09/332,502, entitled, xe2x80x9cSystem and Method for Intrusion Detection Using a Time Domain Radar Array,xe2x80x9d and U.S. patent application Ser. No. 09/332,503, entitled, xe2x80x9cWide Area Time Domain Radar Array,xe2x80x9d both filed Jun. 14, 1999, both of which are assigned to the assignee of the present invention, and both of which are incorporated herein by reference.
Basic impulse radio transmitters emit short pulses approaching a Gaussian monocycle with tightly controlled pulse-to-pulse intervals. Impulse radio systems typically use pulse position modulation, which is a form of time modulation where the value of each instantaneous sample of a modulating signal is caused to modulate the position of a pulse in time.
For impulse radio communications, the pulse-to-pulse interval is varied on a pulse-by-pulse basis by two components: an information component and a pseudo-random code component. Unlike direct sequence spread spectrum systems, the pseudo-random code for impulse radio communications is not necessary for energy spreading because the monocycle pulses themselves have an inherently wide bandwidth. Instead, the pseudo-random code of an impulse radio system is used for channelization, energy smoothing in the frequency domain and for interference suppression.
Generally speaking, an impulse radio receiver is a direct conversion receiver with a cross correlator front end. The front end coherently converts an electromagnetic pulse train of monocycle pulses to a baseband signal in a single stage. The data rate of the impulse radio transmission is typically a fraction of the periodic timing signal used as a time base. Because each data bit modulates the time position of many pulses of the periodic timing signal, this yields a modulated, coded timing signal that comprises a train of identically shaped pulses for each single data bit. The impulse radio receiver integrates multiple pulses to recover the transmitted information.
In a multi-user environment, impulse radio depends, in part, on processing gain to achieve rejection of unwanted signals. Because of the extremely high processing gain achievable with impulse radio, much higher dynamic ranges are possible than are commonly achieved with other spread spectrum methods, some of which must use power control in order to have a viable system. Further, if power is kept to a minimum in an impulse radio system, this will allow closer operation in co-site or nearly co-site situations where two impulse radios must operate concurrently, or where an impulse radio and a narrow band radio must operate close by one another and share the same band.
In some multi-user environments where there is a high density of users in a coverage area or where data rates are so high that processing gain is marginal, power control may be used to reduce the multi-user background noise to improve the number of channels available and the aggregate traffic density of the area.
Wireless communication systems such as impulse radio communication systems that convey coded message transmissions require a close synchrony between receiving stations and received transmitted signals in order that the intelligence contained in the transmitted messages may be properly understood at the receiver station. A key element in establishing such synchrony is the timing system by which appropriate sampling of the received transmitted signal is carried out.
There is a need for an apparatus and method for effecting synchrony in a wireless communication system that is agile in its response to a received transmitted signal.
There is a need for such an agile synchronizing apparatus and method that is less expensive to implement than the presently available voltage-controlled oscillator (VCO) implementations.
An apparatus for effecting synchrony with a received transmitted signal in a wireless communication system is disclosed. The transmitted signal is encoded according to a predetermined coding scheme and arranged in a plurality of first frames in a first succession having a first period. The apparatus comprises: (a) a timing circuit for generating a trigger signal for effecting periodic sampling of the transmitted signal at a sampling node; the trigger signal and the transmitted signal cooperate at the sampling node to generate a detection indication signal; (b) a reference signal source for generating a reference signal for the timing circuit in a plurality of second frames in a second succession having a second period; an amount by which the first period and the second period differ establishes a base offset; (c) an offset generator device; the offset generator device is coupled with the sampling node and with the timing circuit; the offset generator device receives the detection indication signal. The offset generator device determines a code offset related to elapsed time following a beginning of a respective frame of the plurality of first frames during which an expression encoded according to the predetermined coding scheme may be received. The offset generator device generates a frame offset signal by which to effect adjusting sending the trigger signal by the timing circuit from the second period to adjust synchrony with the transmitted signal. The frame offset signal involves the base offset and the code offset.
The method comprises the steps of: (a) generating a trigger signal for effecting periodic sampling of the transmitted signal at a sampling node; the trigger signal and the transmitted signal cooperate at the sampling node to generate a detection indication signal; (b) generating a reference signal for the timing circuit in a plurality of second frames in a second succession having a second period; an amount by which the first period and the second period differ establishes a base offset; (c) determining a code offset related to elapsed time following a beginning of a respective frame of the plurality of first frames during which an expression encoded according to the predetermined coding scheme may be received; (d) generating a frame offset signal involving the base offset and the code offset; (e) applying the frame offset signal to adjust sending the trigger signal from the second period to adjust synchrony with the transmitted signal.
It is therefore an object of the present invention to provide an apparatus and method for effecting synchrony in a wireless communication system that is agile in its response to a received transmitted signal.
It is a further object of the present invention to provide an apparatus and method for effecting synchrony in a wireless communication system that is less expensive to implement that the presently available voltage-controlled oscillator (VCO) implementations.